If the selected file is inserted into the field, check script trigger on the field.
If not, it is not file/field settings. Seems broken dialog box.
If this occur on the file only, try repair it.
If user navigates to a PDF and right click on it to show menu, the top item "select"
should be bold but may be "open" is.
If so, try these
1. Edit>Preferences>General>"Reset the size and position of all dialog boxes to their defaults."
3.edit somewhere in registry ? I don't know where...